The present invention relates to an optical data recording and reproducing apparatus in which image data signals are recorded on and then reproduced from a recording medium. More particularly, the invention relates to an optical data recording and reproducing apparatus of a three-beam type using a plane parallel plate.
A conventional three-beam type optical data recording and reproducing apparatus is shown in FIGS. 4 and 5. The conventional apparatus includes a light source 1 such as a semiconductor laser for generating a light beam to read data out of an optical disk 100, a diffraction grating 2 for dividing the output light beam of the light source into three light beams, a plane parallel plate 3 for reflecting the three light beams towards the optical disk 100 and for astigmatizing the three signal light beams reflected from the optical disk 100, an objective lens 4 for image-forming the three signal light beams reflected from the plane parallel plate 3 onto a recording surface 101 of the optical disk 100, a concave lens 7 for transmitting the three signal light beams which have passed through the plane parallel plate 3, which increases the distances between those signal light beams, and an optical detector 8 adapted to receive the three signal light beams passing through the concave lens 7.
The operation of the conventional apparatus thus constructed will now be described.
The output light beam of the light source 1 is diffracted by the diffraction grating 2 in three directions corresponding to the zeroth, positive first, and negative first (0-th, +1st and -1st) orders of diffraction and also corresponding to the three light beams described above. The three light beams are reflected by a surface of the plane parallel plate 3 towards the objective lens 4, which focuses the reflected beams onto the recording surface 101 of the optical disk 100. The three light beams are modulated with a variation in reflectivity of the recording surface 101 and converted into signal light beams.
The signal light beams are reflected through the objective lens 4 and the plane parallel plate 3, where they are astigmatized, towards the concave lens 7. The signal light beams are applied to the concave lens 7 at such incident angles that they are focused on a front focal plane 41 (FIG. 5) of the objective lens 4. The concave lens image-forms the signal light beams on the detecting surface of the optical detector 8.
Referring to FIG. 5, without the concave lens 7, the three signal light beams (dotted lines) would focus on the front focal plane 41 with a small distance L.sub.1 therebetween. However, because of the diffusion characteristic of the concave lens 7 (which corresponds to the magnification of the concave lens 7), the three signal light beams are image-formed on two photo-diodes (PD) 82 and 83 and a four-division photo-diode (4D-PD) 81 of the optical detector 8 with a distance L.sub.2 (L.sub.2 &gt;L.sub.1) therebetween.
However, the conventional optical data recording and reproducing apparatus has the following disadvantages. In the conventional apparatus, it is necessary to interpose the concave lens 7 in the optical path to increase the distances between the three signal light beams. But, because of the magnification of the concave lens, the optical path is increased as much as distance L.sub.3 as shown in FIG. 5. Consequently, it is very difficult, if not impossible, to miniaturize the optical recording and reproducing apparatus.
The above-described disadvantages may be eliminated by another conventional optical recording and reproducing apparatus wherein a concave lens is used. In this case, the signal light beams which pass directly through the first surface (i.e., the top side in FIG. 4) and through the second surface (i.e., the bottom side in FIG. 4) of the plane parallel plate 3 are detected with a first optical detector. On the other hand, the signal light beams which pass through the first surface and are reflected by the second surface back through the first surface are detected with a second optical detector.
In the latter conventional optical data recording and reproducing apparatus, the three signal light beams emerging from the first surface of the plane parallel plate advance towards the light source. Hence, the second optical detector must be positioned extremely close to the light source (the distance therebetween being 1 to 15 mm). However, when the second optical detector is positioned so close to the light source, it blocks the output light beam of the light source. This "mechanical interference" is a problem.